High levels of supercooling are not generally required in industrial processes for two related reasons. Firstly, the supercooled state is unstable and spontaneous crystallisation can theoretically occur at any time. Secondly, nucleation from a highly supersaturated system is rapid, leading to small crystal sizes. In many industrial processes, where crystallisation is used as a separation technique, large crystals rather than small crystals are required.
However, although for industrial separation processes crystallisation at high levels of supercooling is seen as a disadvantage, there are areas, especially within the food industry, where the small crystals resulting from high nucleation rates are an advantage.
It would therefore be advantageous to be able to achieve a high degree of supercooling in a liquid or solid. To date attempts to achieve a high degree of supercooling have been unsuccessful. In academic studies high levels of supercooling have been created by rapidly freezing very small samples of material. At industrial scale, however, any method for producing, handling and utilising supercooled materials must ensure that crystallisation does not take place until required. Thus supercooling to approximately only 1.degree. C. below the equilibrium freezing temperature of the material has been achieved.
The state of the art is confirmed by the teaching given in the latest edition of the book "Crystallisation" (J. W. Mullin, 1993, Butterworth-Heinemann). This book states that supercooling below about 0.5.degree. C. is not possible in a stirred system.
We have now observed that by determining the metastable limit temperature for a system, we are able to controllably produce materials cooled close to the metastable limit temperature without inducing crystallisation. Thus supercooling to temperatures greater than 1.degree. C. below the equilibrium freezing temperature of the material can be achieved.
The term "metastable limit temperature" is a well known term within the art. A suitable method for measurement is described in "Crystallisation", J Mullin, Butterworth 1972, p.178 & 179 and FIG. 6.4.